Thermoelectric conversion technology is one that takes advantage of the Seebeck effect of a material to accomplish direct conversion of thermal energy to electric energy or the Peltier effect of a material for refrigeration. Characterized by no operating components, high reliability, long life, environmental friendliness, etc., the technology may be used in a wide variety of fields such as power generation using residual heat, aerospace power supply, medical refrigeration, home appliances for refrigeration and the like. Thermoelectric conversion efficiency is mainly determined by the dimensionless performance index ZT of the material (ZT=S2σT/κ, wherein S stands for Seebeck coefficient, σ for electric conductivity, κ for thermal conductivity, and T for absolute temperature). A higher ZT value of the material means a higher thermoelectric conversion efficiency.
When a p-type thermoelectric material and an n-type thermoelectric material are combined to form a thermoelectric device, the thermoelectric conversion efficiency of the device is closely related to the temperature difference and the average Z value between the low temperature end and the high temperature end. The maximum thermoelectric conversion efficiency may be figured out as follows:
      η    max    =                              T          h                -                  T          l                            T        h              ⁢                                        (                          1              +                              Z                ⁢                                  T                  _                                                      )                                1            2                          -        1                                          (                          1              +                              Z                ⁢                                  T                  _                                                      )                                1            2                          +                              T            l                                T            h                              wherein T=(Th+Tl)/2 is the average temperature, and Z=S2σ/κ (represents the average Z value of the p-type semiconductor and the n-type semiconductor in the whole temperature range Tl-Th from the low temperature end to the high temperature end. Therefore, with respect to the enhancement of the efficiency, materials having higher ZT values across the whole temperature range are desirable.
Owing to its superior electric transmission property and relatively low thermal conductivity, filled skutterudites are considered to be an ideal thermoelectric conversion material for use at medium to high temperatures (500-800K). The crystal lattice thermal conductivity of such a material may be decreased by small-radius atoms filled in an icosahedron cage structure where the atoms can form weak bonds with the surrounding atoms of phosphorus family so that the disturbance effect is incurred and phonons are scattered effectively. Thus, the type and the amount of the filled atoms are typically modified to optimize the thermoelectric property of a material. However, the decrease of the crystal lattice thermal conductivity is generally accompanied by substantial increase of carrier concentration, leading to degradation of the Seebeck coefficient of the material. Therefore, it is difficult to improve the thermoelectric property of filled skutterudited further solely by changing the type and the amount of the filled atoms.
A second phase is commonly introduced as a scattering center of phonons into a thermoelectric matrix, so that a maximum decrease of the crystal lattice thermal conductivity of the material can be achieved. Generally, the second phase is a second phase of nanoparticles. Since phonons are featured by a relatively broad frequency distribution, the second-phase particles of different size may scatter different phonons of corresponding waves effectively. It is generally believed that particles of 50-300 nm in particle size have no substantial impact on the transmission characteristics of carriers. However, when the second-phase particles are fined to 10-20 nm in size, the second nanophase may scatter the carriers to such an extent that electrons of low energy are filtered off. Electrons of low energy contribute relatively low to the Seebeck coefficient of a material, they can lead to significant increase of the Seebeck coefficient after they are scattered fiercely. On the other hand, the total thermal conductivity has no change or has slight decrease. As a result, the ZT value of the material is increased. The filtration of low-energy electrons by nanoparticles is shown schematically in FIG. 1.
In order to achieve an ideal effect in scattering phonons and electrons, second-phase particles can be introduced into a filled skutterudite matrix material homogeneously. A second nanophase is generally introduced by one of the following processes.
(1) Mechanical mixing. Ball milling was used by Katsuyama et al. to prepare CoSb3/FeSb2 and CoSb3/NiSb composite materials, which had better properties than CoSb3 composite material (J. Appl. Phys., 88, 3484, 2000. J. Appl. Phys., 93, 2758, 2003). Although this process has the advantage of being simple, the second nanophase doped therein is generally characterized by a rather large size, and homogeneous distribution is difficult to be achieved in nanoscale. Thus, the second nanophase has limited contribution to the scattering of phonons.
(2) Oxidizing a component of the thermoelectric matrix. According to Kusakabe et al., a thin oxide layer was obtained on the particle surface of CoSb3 powder by oxidation to lower the thermal conductivity of the matrix and increase the Seebeck coefficient (U.S. Pat. No. 5,929,351, Jul. 27, 1999). However, it is quite difficult in practice to adjust process parameters such as temperature, oxygen partial pressure and the like to control the oxidation of the thermoelectric matrix precisely. In other words, it is by no means easy to choose process conditions suitable for forming an oxide layer on the granular surface of the matrix without sacrificing its electric properties.
(3) Forming a second nanophase, for example Sb, via in-situ precipitation (Appl. Phys. Lett., 92, 202114, 2008). Although this process can promote homogeneous dispersion of nanoscale Sb in the matrix, Sb tends to evaporate in use due to its low melting point (˜631° C.) and high vapor pressure (0.01 kPa). Furthermore, as a metal phase, Sb brings to the composite material a carrier concentration much higher than is suitable for giving the material optimal thermoelectric property, so that the electric transmission property of the composite material is degraded. Despite that the in-situ process can promote homogeneous dispersion of nanoparticles in the matrix, it is difficult currently to find a suitable component as well as a suitable process to form in-situ a stable second nanophase.
According to Johnson et al. (U.S. Pat. No. 5,994,639), the thermoelectric property of a skutterudite material having a meta-stable structure of super crystal lattice may be improved. However, this typical lamellar structure can be generally obtained in a two-dimensionally structured material (e.g., thin film) rather than three-dimensionally one.
In view of the foregoing, there is a need in the art for a nanoscale filled skutterudite composite material having a stable second phase of particles to impart to it superior thermoelectric property and a process for preparing the same.